Calculation of radiated power density in CST

Hi, I have to calculate the power density radiated from a grid of holes in a waveguide.
I have set up the time domain simulation like this:
Waveguide is made of PEC, open boundary conditions (add space). Two waveguide ports.
I defined a plane at the distance where I want the calculation and set a power monitor at the interest frequency.
I have put the source at port 1, mode 1. I have tried to use both the default signal and a sine step. I used the post processing to scale this signal to 100 W.
Then I use the postprocessing to get the maximum value of the power on the plane. I assume this is in W/m2. My problem is that I get a value which is 10 times smaller than the measured one. Somebody has suggestions? Am I doing something wrong?

Hello,
As I can imagine, for the real measurements you use only one feed at the end of waveguide? Try to delete or turn off the second waveguide port in CST.